Burning incense, popular in places of worship and in people's homes, could be a cancer risk.

Researchers in Taiwan found that the smoke produced by burning incense is laden with cancer-causing chemicals.

Levels of one chemical believed to cause lung cancer were 40 times higher in a badly ventilated temple in Taiwan than in houses where people smoke tobacco.

Incense burning also creates more pollution than road traffic at a local intersection.

Ta Chang Lin, of the National Cheng Kung University in Tainan told New Scientist magazine: "We truly hope that incense burning brings only spiritual comfort, without any physical discomfort.

"But there is a potential cancer risk. We just cannot say how serious it is."

Air samples

Dr Lin's team collected air samples from inside and outside a temple in Tainan City and compared them to samples at a traffic intersection.

Incense burning caused more pollution than traffic

Inside the temple, they found very high concentrations of polycyclic aromatic hydrocarbons (PAHs), a large group of highly carcinogenic chemicals that are released when certain substances are burnt.

Total levels of PAHs inside the temple were 19 times higher than outside and slightly higher than at the intersection.

A PAH called benzopyrene, which is thought to cause lung cancer in smokers, was found in very high quantities inside the temple.

Levels were up to 45 times higher than in homes where residents smoked tobacco, and up to 118 times higher than in areas with no indoor source of combustion, such as cooking fires.

Dr Lin said: "The concentrations of PAHs inside the temple depends on how many visitors come to worship that day.

Ceremonies

"During some major ceremonies, hundreds or even more than a thousand sticks are burnt at the same time.

"Sometimes the visibility is so low you can't see clearly across the room. We are concerned for the health of workers or keepers in the temples."

Dr Lin's team also tested for pollutants known as total suspended particles.

Concentrations of TSPs inside the temple were three times higher than at the traffic intersection and 11 times higher than outside the temple.

Brad Timms, a science information officer for the Cancer Research Campaign, told BBC News Online: "We know from previous research that industrial workers exposed to PAHs have an increased risk of lung and bladder cancer, and PAHs are also a recognised cause of occupational skin and scrotal cancers.

"The risk of cancer from burning incense will depend on the levels of PAHs given off from this source and the length of time of exposure, and clearly more research is needed to determine these factors.

"In the meantime workers in temples could minimise exposure to themselves and their worshippers by improving ventilation."